Fix and improve ServiceWorker configuration
The service worker now only prefetches JS files for ES2015 modules. ES5 files will be lazy loaded only by legacy browsers. Futhermore, WOFF2 webfonts which are already used on the home page are prefetched (Material icons, 400 and 500 font weight versions of Roboto).
These changes improve performance for current browsers at the expense of performance for legacy browser which do not support ES2015 or WOFF2.
Closes: #695 (closed)